DebuggingHelpers: Pass mkspec on command line

We didn't pass actually the targetMode and mkspec to qmake. E.g.
on Mac, debugging helpers therefore couldn't compile if the default
qmake mkspec is xcode.

Task-number: QTCREATORBUG-5806
Change-Id: I575012386983e1e285a500f99a34b7f21dcba29d
Reviewed-on: http://codereview.qt.nokia.com/2947


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>